778 



Mr. F. Twyman on an Interferometer 



reflected. One part passes through the prism L in the same 

 way as in actual use, and, being reflected by the mirror F, 

 passes back through the prism to the plate K. The other 

 part of the light is reflected to the mirror Gr and back again 

 to the plate K. Here the separated beams re-combine, and, 

 passing through the lens E, each forms on the eye placed 

 somewhat beyond the aperture in the diaphragm P an image 

 of the hole in the diaphragm C. 



fig. 1. 



Diagram of Prism Interferometer. 



When the mirrors are adjusted, interference-bands are seen 

 which form a contour map of the glass requiring to be re- 

 moved from the prism face in order to make its performance 

 perfect. Fig. 2 represents in diagram a typical map, where 

 Q represents the highest point of a " hill." The procedure 

 in such a case is to mark out the contour lines on the surface 

 of the prism with a paint-brush and then to polish first on the 

 region Q, subsequently extending the area of polishing first 



